1. Field of the Invention
This invention pertains to the field of viscosity measurement. In particular, to the field of capillary viscometers, specifically, a novel three-capillary viscometer used to measure the viscosity of a solution.
2. Description of the Prior Art
The measurement of a fluids' viscosity is a very important part of Liquid Chromatography (LC). Particularly, in Size Exclusion Chromatography (SEC) a viscometer detector in combination with a concentration detector, provides important information about molecular weight in polymer analysis.
The most basic capillary type viscometer is the single capillary viscometer. It is based on the idea that when a fluid goes through a capillary tube, the pressure drop across the capillary is proportional to the fluid viscosity and flow. They are related according to Poiseuille's law: EQU P=R.multidot..eta..multidot.Q R=8/.pi..multidot.L/r.sup.4
Where:
P Pressure drop across capillary PA1 R Capillary geometrical restriction PA1 .eta. Viscosity of solution PA1 Q Solution flow rate PA1 L Capillary length PA1 r Capillary inside radius
In practice, a pressure transducer is placed across the capillary to measure the pressure drop. If the flow is constant, the pressure is proportional only to the solution viscosity. This viscometer is very simple but has the disadvantage that the pressure output is also proportional to the solution flow rate. Therefore, any small disturbance in the fluid flow through the capillary generates a pressure drop of the same magnitude, or even greater, than the pressure drop caused by the solution viscosity.
The flow through the capillary depends primarily on the pumping system flow, and on the temperature changes in the whole system. All the slow and fast flow disturbances created by the pump are clearly detected by the viscometer. Also, any temperature change anywhere in the chromatograph creates solution expansions and contractions, that in turn create flow disturbances that are also detected by the viscometer. Even the viscosity changes due to the injected sample, can potentially create a flow disturbance. If the pumping system cannot react quickly to the varying pressure load created by the sample passing through the system, it also causes flow disturbances in the viscometer.
All these issues impose extremely rigid requirements on the pump and the whole system, to obtain good performance out of the single capillary viscometer. The pumping system should deliver an extremely precise and constant flow, free of any slow drifts or fast transients as those normally associated with the repetitive pump action. The entire system should be maintained at constant temperature to eliminate flow errors due to temperature changes. The volumes and tubing sizes in the system should be carefully considered to prevent flow disturbances due to the sample viscosity.
There are several types of capillary viscometers. The single-capillary viscometer discussed above was originally described in U.S. Pat. No. 3,837,217. Another single capillary design is shown in U.S. Pat. No. 4,286,457. The single capillary design suffers from the major drawbacks described, including temperature sensitivity, sensitivity to minor pump fluctuations, and general intolerance to minor system disturbances such as injection.
A multiple capillary design is described in U.S. Pat. No. 4,463,598 (Haney). Haney discloses a bridge-type viscosity measuring device having two separate branches. Each branch has two capillaries arranged in series. The branches are connected at the top and bottom by common input and output lines. A bridge having a dead-ended pressure transducer connects across the branches in their middle between the first and second capillaries, thereby measuring differences in pressure across the two branches at those points. Under normal operation (viscosity the same in both branches) there is fluid flow down both branches encountering the same resistance and hence no pressure difference. In operation, a fluid of different viscosity is introduced into one branch, and as it enters the capillary a pressure differential begins to build until a maximum is reached when the sample is entirely within the capillary. The pressure difference is then measured via the transducer and mathematical operations give relative viscosity of the two solutions. A key drawback is the necessity to balance the capillaries so that they are substantially equal in resistance.
U.S. Pat. Nos. 4,627,271 and 4,578,990 to Scot D. Abbott and Wallace W. Yau (Abbott et al.) describe a differential pressure capillary viscometer which may be used to measure viscosity independent of flow rate and temperature fluctuations. These patents disclose a viscometer in which a solvent is pumped from a reservoir into a system comprising a solute injection valve upstream of two capillary tubes which are separated by a large depository column which is used to trap solute, so that only solvent flows through the second capillary tube. Changes in pressure across both capillary tubes are measured and converted into electrical signals, which are fed to a differential logarithmic amplifier. The output signal of the differential logarithmic amplifier is related to the natural logarithm of the relative viscosity. Both the inherent and intrinsic viscosities may be related mathematically to the experimentally measured value for the relative viscosity. Although the apparatus disclosed in these patents provides a viscosity measurement which is independent of flow rate and temperature fluctuations, it is, however, sensitive to fast flow transients or high frequency flow pulses, like those caused by the pumping system or the sample injector.
Yau, U.S. Pat. Nos. 4,793,174, and 4,876,882 disclose a similar apparatus, with the exception that the large depository column is eliminated, and a small separation volume is placed between the two capillaries instead. This effectively eliminates the delay associated with the large depository column, while retaining independence of flow rate and temperature fluctuations. However, the apparatus described in these patents require closely matched capillaries for best performance, and they are also sensitive to fast flow transients and high frequency flow pulses.
Furthermore, all the previous designs use pressure transducers with "dead-end" connections, that must be purged for correct operation. This creates additional problems as described below. Therefore, all these designs have some deficiencies and to date none show robust and accurate operation. There is a need for an improved design for capillary viscometers.